With the progress of a computer image-processing technique, a method of directly writing images on a photosensitive layer, by light irradiation corresponding to digital signals, has recently been developed. An intense interest has been shown toward a computer-to-plate (CTP) system in which images are directly formed on a photosensitive lithographic printing plate, without outputting the images onto a silver-salt mask film, by employing said method for a lithographic printing plate precursor.
A negative photosensitive lithographic printing plate for CTP system includes, for example, a so-called photopolymer type photosensitive lithographic printing plate in which a photosensitive layer, made mainly of a photocurable resin, is formed on a substrate. In the photopolymer type photosensitive lithographic printing plate, a photopolymerization initiator contained in the photosensitive layer is excited, on exposure, to form a radical whereby the photocurable resin is crosslinked and insolubilized to form an image.
By the way, when the crosslinking reaction of the photocurable resin proceeds, the photosensitive layer is necessarily contracted. The contraction may decrease the adhesion between the substrate and the photosensitive layer of the photosensitive lithographic printing plate thereby to deteriorate printing characteristics. Therefore, as described in Japanese Unexamined Patent Publication (Kokai) No. 11-143082, there is proposed a technique in which fillers such as silica particles are incorporated into the photosensitive layer thereby to increase the strength the photosensitive layer and to reduce the contraction of the photosensitive layer.
However, even when these fillers are used, it is still difficult to sufficiently secure adhesion between the photosensitive layer and the substrate, thus lowering the run length of the photosensitive lithographic printing plate.